The present invention relates to a Photoplethysmography(PPG) device and the method thereof. More particularly, the present technology relates to a pulse oximeter device and the method measuring accurate Photoplethysmography(PPG) signals, reducing the artifacts occurred by the patient motion.
Photoplethysmography (PPG) is an electro-optic technique to measure the pulse wave of vessels.
The measuring apparatus for PPG is the pulse oximeter, which considers the relative absorption of Hemoglobin and Oxyhemoglobin to non-invasive measure of arterial oxygen saturation(SpO2) using the dual-wavelength illumination(LED), and the signal measured by PPG is called photoplethysmographic signal.
The arterial oxygen saturation (SpO2) from PPG is a fundamental parameter to diagnose the heart and lung function, the information on the idiosyncrasy in circulating system, the status of heart vessels, and hypoxia, etc. Especially, PPG signal is one of vital signs, basic signal to measure vital parameters of a case, and therefore PPG is used as an important monitoring aid at casual wards and intensive care units.
The major factors limiting the accuracy and the general applicability in practical pulse oximetry, is low perfusion states or artifact.
The artifacts added to the oximeter limit the accuracy of measured PPG signals, are mainly occurred by the patient motion and ambient artifact, causing artifact corruption. The ambient artifact is occurred by injected environmental lights.
The ambient artifact, can be regarded to be less serious, can be appropriately removed through subtraction process, to adopt multiplexing technique against the ambient signals measured independently to restore PPG signal.
But, the motion artifact, caused by changes in optical probe coupling, patient anatomy, and optical properties of tissue due to geometric realignment, and complex combinations of all these effects, considerably deteriorates the shape of PPG signals and the value of SpO2.
Moreover, the difficulty to reduce motion artifact can be aroused from that PPG and motion artifact signals mostly occupy the frequency band and time domain, same to original signal. Hence, the motion artifact reduction has been regarded as one of important issues to be handled for accurate clinical measurement.
Some methods have been researched to remove the motion artifact from PPG signals. These methods are the modeling method and three-wavelength method. Typically, the motion artifact detection method identifies the motion artifact from time sequenced PPG signals for efficient artifact concealment; and the modeling method imitates physical process of probe coupling to remove the motion artifact by the inversion of modeled artifact; and three-wavelength method subtracts the motion artifact by use of additional wavelength comparing with dual wavelengths generally used in PPG device. However, these conventional methods still contains limitation in performance for some specific noise, or limitation in mathematical modeling.
As further invention is still required to improve the performance of motion artifact rejection without adoption of fake concealment under dual wavelengths constraint, in this invention, the motion artifact reduction method under the constraint of dual wavelengths measurement was newly invented from two simplified observations.
The signal analysis to reduce motion artifact, basically utilizes independent component analysis(ICA). The basic motivation of ICA is assuming that original PPG signal is from heart pulsation, and motion artifact is another physical factor. Under the assumption, it is clearly possible to model the PPG signal measured with artifact into independent components, original PPG signal and the motion artifact. Based upon the model, the original PPG signal and the motion artifact can be separated through ICA, adopting subtraction technique to minimize the influence of ambient light. But, the practical factors causing motion artifact, also influences on the measured PPG signal, questions are raised in the given assumption, and can be recognized to have difficulties in restoring the original PPG signal by ICA technique.
Hence, the present invention discloses a new Photoplethysmography(PPG) device and the method therefor, to extract purely original PPG signal reducing motion artifact effectively to enhance the performance of conventional ICA method, combining various preprocessing techniques.
The present invention also discloses a new processing technique to reduce/eliminate motion artifact in measured PPG signal. For that; before to analyze independent components, from the periodicity of PPG signal, the period of PPG signal is extracted by using auto-correlation function; rearranges signal according to the extracted period by interleaving process; eliminates noise by time low-pass filtering; and increase the independence between PPG signal and motion artifact by innovation processing.